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This is an edited and condensed excerpt from The Modem
Reference, written by Michael A. Banks and recommended by the
Associated Press, The Smithsonian Magazine, Jerry Pournelle in
Byte, et al.
The right to reproduce this article is granted on the
condition that all text, including this notice and the notice at
the end of the article, remain unchanged, and that no text is
added to the body of the article. Thanks! --MB
Copyright (c), 1988, 1989, 1990 Michael A. Banks
All Rights Reserved
(From Chapter 3)
DATA TRANSMISSION MODES
Now that you have a good understanding of the basics of data
transmission (you did read the companion file to this one), we'll
examine the major modes such transmission can take.
The binary signals that must be transmitted for each data
character can be sent simultaneously (parallel transmission) or
one at a time (sequentially, in serial transmission).
Parallel Transmission
Parallel data transmission involves sending all of the bits
that make up a character at the same time, using a separate wire
for each bit. In parallel transmission, each bit in a character
is sent as shown in Figure 3.13 in THE MODEM REFERENCE. This
involves eight data bits, and thus eight wires.
A ninth wire is used to send a special signal called a
"strobe" or clock. This signal is sent after all 8 data bits are
sent, to inform the receiving device that all bits have been
sent.
Data transmitted in this manner is not modified before or
after transmission.
Parallel transmission is commonly used to send data from a
computer to a printer or another device located relatively
nearby. Longer runs for parallel transmission are not very cost-
effective, because of the number of wires involved as well as the
more complex transmission equipment required.
Serial Transmission
When data is transmitted in serial format, each character's
bits are sent individually, over a single wire.
Some modification is required to prepare data for serial
transmission, because the data a computer sends to its serial
port is in parallel format. The serial port must convert this
parallel data to serial format (using what are called shift
registers). Among other things, this involves "lining up" the
data bits in series, beginning with the lowest-value bit. At the
receiving end, the serial port has to sort out the bits and
return them to parallel format.
Figure 3.14 illustrates how serial data transmission works.
When compared with parallel transmission, serial data
transmission would appear to be at a disadvantage. It requires
special circuitry and other accommodations, and is somewhat
slower than parallel transmission. Too, serial data must be
synchronized, which sometimes means adding bits to each
character.
However, using one wire to send data is far less costly than
using eight. Furthermore, the standard voice telephone system
(noted earlier as the medium used for most telecomputing) can't
accommodate parallel data transmission because it provides only
two wires. So, serial transmission is the norm for telecomputing
when two-wire (or four-wire), voice-grade telephone lines are
involved.
Synchronization. The sending and receiving systems must be
synchronized during serial transmission, so that individual bits
and full characters can be recognized and sorted out by the
receiving system in the same pattern as they were sent. Without
synchronization, the receiving system would perceive data as a
seemingly-endless series of unrelated binary digits, with no
indication of where one character ended and the next began.
There are several modes of synchronization available.
You'll probably encounter only two of them--synchronous and
asynchronous. In data communication via microcomputer, these are
virtually the only synchronizing modes in use.
Synchronous transmission. Synchronous data transmission
involves a continuous and consistent (in time) transfer of data.
The duration of and time between each bit or character sent is
preset by the sending and receiving systems. This provides a way
for the receiving system to know when to look for each character
as well as how long it will take to transmit a character. Modems
that can be synchronized in this manner are called synchronous
modems.
Depending on the protocol used, time synchronization is
usually effected by a special information signal that precedes a
data transfer or by information contained in a group of bytes
(called a block). This signal enables the systems to synchronize
their internal clocks, and may come from the computer or the
modem.
For reasons discussed in the next section, synchronous
transmission can be up to 20% faster than asynchronous
transmission. But it requires more expensive equipment. (The
expense is largely due to the additional circuits and programming
required to synchronize timing.)
Asynchronous transmission. Asynchronous transmission is a
data transfer mode that notifies a receiving system when each
character begins and ends by "framing" bytes with additional
bits. These extra bits include a start bit and a stop bit, which
are discussed in detail in the next section (DATA TRANSMISSION
PARAMETERS). Modems that operate in asynchronous mode are called
asynchronous modems, or asynchronous communications adaptors.
As noted a few lines back, asynchronus transmission is a bit
slower than synchronous transmission. (One reason for this is
the presence of start- and stop-bits.) The advantages of
asynchronous transmission far outweigh the loss in speed,
however. Because no timing circuits are required, asynchronous
modems are less expensive than their synchronous counterparts.
Too, they are generally more "tolerant" of line noise, delays in
sending, and so forth. So, you'll find the majority of modems
used with microcomputers to be asynchronous; because of this the
majority of dial-up systems (BBSs and online services) also
operate in asynchronous mode.
#
There are modems that operate in both synchronous and
asynchronous mode. However, unless you are communicating with
mainframe computers, Local Area Networks (LANs), or other systems
that use synchronous data transfer, your telecomputing needs will
probably dictate an asynchronous modem.
Digital Data Transmission
When a data communications system is not restricted by the
frequency bandwidths and other limitations of a voice-grade
telephone system, digital transmission can be used. In digital
data transmission, data does not have to be translated to analog
format. Instead, it is sent as binary signals, just as it would
be during a direct data transfer between two computers using a
null-modem cable.
This kind of transmission accommodates extremely high baud
and bps rates, but there are tradeoffs. Digital transmission
requires special interfaces or modems, and is generally used only
with digital dial-up networks. Such networks use special
conditioned lines and switching equipment to maintain the
integrity of the digital signals they carry, and are extremely
expensive to build and maintain.
DATA TRANSMISSION PARAMETERS
In a very general sense, this book is not unlike a data
communications link. I am communicating certain information and
ideas to you; this book is the vehicle for that communication,
just as the a data link is the vehicle computer-to-computer
communication.
You can understand the information and ideas herein because
I write in a language you understand, and conform to certain
rules of grammar and syntax. (Whether that information is useful
to you or not is irrelevant to the topic; as I said early in the
chapter, we are more concerned here with the form of information
than with its content.) In short, we agree as to the parameters
of our communication; thus we communicate.
Computers must likewise agree on communications parameters.
There are several important data communications parameters that
define the operation and characteristics of a data communications
system. They include the following:
* the number of bits transmitted for each character
* how (or if) data is checked for errors
* when each character transmission begins and ends
* whether your system or the remote system displays what
you type on your screen
* the speed at which transmission takes place
The parameters are data bits, parity, start- and stop-bits,
duplex, and speed.
As you probably know, computers are quite literal devices,
and, unlike humans, do not tolerate deviations from rules. So,
each of the parameter settings must be set to the same value on
both systems.
Parameter settings are altered via commands to your modem or
communications software, and it is normally up to you to match a
host system's communications parameters.
Now, let's take a look at each of the parameters.
Data Bits
The phrase "data bits" refers to the number of bits (binary
digits) sent for each data character transmitted, not counting
parity, start, or stop bits--which are explained in a few
paragraphs. This parameter is sometimes called "data word
length" or "character length." Depending upon what's being
transmitted, either seven or eight bits are used. The number of
data bits used is set by a software command; the computer's
serial port does the actual configuring of data before it is
transmitted.
Each of the first 128 characters in the ASCII character set
can be represented by seven-digits binary numbers (that is, the
binary counterparts of the numbers 0 through 127 are each
composed of seven or fewer digits). The majority of data
communication uses only the first 128 ASCII characters, because
all systems have these characters in common. So, much data
transmission takes place using the 7-bit ASCII characters 0
through 127.
Seven bits are used whether or not the character actually
uses seven places/digits (i.e., leads off with a "1" in the
seventh place, as is the case with 1100010--a decimal 98, which
is the ASCII number for a lower case "b"). When necessary, 0's
are added to fill out the binary number to a full seven digits--
as with 111111 (a decimal 63, the question mark character on the
ASCII table.) This binary number is sent with a leading zero,
thus: 0111111.
If special characters from an extended ASCII character set
are used (ASCII numbers 128 through 255), eight data bits must be
used. This is because each of the binary numbers 128 through 255
are composed of eight binary digits, each leading off with a 1.
8-bit transmission must be used with special binary file
transfer protocol transfers, too, as you'll learn when we discuss
these protocols.
Parity
Parity is an extremely simple (and not quite perfect) way
for a receiving computer to determine whether a data character
has been properly received. (This error checking, by the way, is
quite apart from special error-checking file transfer protocols,
which we'll get to later in the book.)
There are two types of parity checking: Even/Odd and
Mark/Space. Almost all communications software packages allow
you to change the parity setting (the choices available depend
upon the type of parity checking in use). More systems use
Even/Odd parity than Mark/Space parity.
Even/Odd parity. Even/Odd parity works by checking whether
the sum of the 1s in a string of bits is even or odd. The
settings available with most communications programs are "Even,"
"Odd," or "None." (When "none" is selected, parity checking is
effectively "turned off.")
When Even/Odd parity checking is used, the sending computer
adds up the 1's in each binary number it sends, and determines
whether their sum is odd or even. It then adds an extra bit,
known as the parity bit, to the character at its serial port.
(As with other data bits, the parity bit can be either 0 or 1.)
The purpose of the parity bit is to make the sum of the 1s in the
character even or odd, as called for by the current parity
setting.
For example, if the letter "A" (1000001) is sent under even
parity, a 0 is added as the parity bit--making it 10000010. This
is because the sum of the 1s in the binary number is already
even. If the same data character is sent under odd parity, a 1
is added as the parity bit, so that the sum of the 1s in the
character would be odd. (Thus, the character would be
transmitted as 10000011.)
The receiving computer checks the sum of the 1s--counting
the parity bit as well as the bits making up the character. If
the total doesn't agree with the parity setting (even or odd), it
signals the transmitting computer to resend the character
(provided its software supports this). The extra parity bit is
"stripped" and ignored when it is sent to the software from the
serial port for "end use."
Most systems that use 7-bit communication use even parity,
while systems that use 8-bit communication use no parity.
(Parity-bit checking is theoretically possible with 8-bit
communication, but not practical.)
Although it is generally effective, parity checking has its
shortcomings. It can detect a "dropped" or garbled bit only if
the bit is a 1. And if more than one 1 is dropped, the parity
check may sometimes pass the defective character anyway, because
the sum of the 1s still agrees with the parity setting (as when
two 1s are dropped when the parity is set to "even").
A parity bit is transmitted at the end of a seven-bit
character as shown in Figure 3.15.
(NOTE: As I noted during the discussion
of binary signals earlier in this chapter, a
character's data bits are transmitted in
reverse order. This is so both the sending
and receiving serial ports can calculate the
number of 1s in each character as the
character is sent.)
Mark/Space parity. Mark/Space is even simpler than Even/Odd
parity checking. When Mark/Space parity is used, the sending
system puts either 1 or 0 in the parity bit's "slot." The
receiving system looks for the parity bit to be a 1 or a 0, as
appropriate with active setting. When set to "Mark," the parity
bit must be 1; when set to "Space," the parity bit must be 0.
As with Even/Odd parity, parity is checked, and parity bits
added and stripped, at the systems' serial ports.
Start- and Stop-Bits
When data are transmitted in asynchronous mode (as will be
the case in any telecomputing applications you are likely to
encounter), there may be varying time lapses between the
transmission of characters. Thus, the receiving modem cannot
rely on a set time frame between the end of one character and the
beginning of another to know when to look for the new character.
This problem is solved by the use of special bits--called
start bits and stop bits--added to the beginning and end of a
data character.
Start bits. A start bit marks the beginning of a new string
of bits (i.e., a new character).
A start bit is always a 0. This is because the normal
"state" of a modem when it is not sending data is the equivalent
of a digital 1, or negative voltage. When a 0, or positive
voltage comes through after a period of no transmission, the
receiving system knows that a new character transmission has
started.
Stop bits. To let the receiving modem know when the
transmission of a string of bits is complete, the sending
computer adds a stop bit to the end of a character (following the
parity bit, if any). The stop bit provides a timing "bench mark"
whereby the receiving modem knows when a complete character has
been sent.
This bench mark involves the baud rate, which is tied to the
time it takes for the sending system to send each bit (i.e., make
a change in state from high to low, or 0 to 1). The timing of a
stop bit is entirely dependent upon the baud rate.
Most systems use either one or two stop bits; one stop bit
is by far the most common.
Figure 3.16 shows the placement of the start and stop bits
in a transmitted data character.
Duplex
Duplex is one of those unfortunate terms that has two
meanings. It has one meaning as a parameter setting, and another
as a transmission mode. Worse, there are several other terms
used in place of duplex when it is used as a parameter. (Don't
worry--I'll untangle this mess for you. Of course, if you've
been computing for a while, this sort of dichotomy is not new to
you, I'm sure.)
As a parameter or a mode, duplex has two settings: full-
duplex and half-duplex.
Duplex parameter. As a parameter setting, duplex is used to
determine whether the computer you've dialed up echoes the
characters you type at your computer's keyboard back to your
computer for display. When you set your software to full-duplex,
it expects the remote system to echo characters back to you.
When you set your software to half-duplex, it echoes the
characters you type. Naturally, the remote system's duplex
setting should agree with your system's.
Looking at it another way, a system set to full-duplex is
responsible for echoing characters back to its counterpart, but
not to its own screen. A system set to half-duplex echoes no
characters back to the remote system, but echoes characters to
its own screen.
Obviously, when the remote system is set to full-duplex,
your system should be set to full-duplex. Otherwise, both your
computer and the remote system will be echoing characters to you,
and you'll see double characters, lliikkee tthhiiss..
Conversely, if the remote system is set to half-duplex, and your
system is set to full-duplex, no characters are echoed to your
screen and you will not see what you type. So, duplex modes must
match.
In this context, duplex is more properly referred to as
echo, or echoplex, with local echo/remote echo or echo/noecho as
the available settings. (A very few software packages call
duplex mode, with the half and full as the available settings.)
Most BBSs and dial-up systems operate in full-duplex, which
means that you should set your system to full duplex (or to
remote echo or noecho) as well. Many systems that operate in
half-duplex (such as GEnie) can be changed to communicate with
your computer in full-duplex by a simple online command.
Duplex mode. As a transmission mode, duplex determines
whether or not data is transmitted in two directions at once.
In full duplex mode, data flows in both directions at once
(two different carrier signals are used).
In half duplex mode, data flows in only one direction at a
time. When half duplex mode is in effect, each system must
signal the other when it has finished transmitting data and is
ready for a reply. All of this back-and-forth happens at the
speed of light, so of course you'll notice no perceptible delay
at turnaround.
Duplex mode is dependent upon modem capability. Some modems
operate at either half or full duplex, while others operate at
only half duplex or only full duplex. The majority of modem data
communication--especially communication involving BBSs and online
services--takes place in half duplex transmission mode (data is
transmitted in one direction at a time), due to the limitations
imposed by telephone line bandwidth. Leased line modems, on the
other hand, typically communicate at full duplex (data
transmitted in both directions at once).
#
(A reminder: When you set duplex with the majority of
software packages, you aren't changing the transmission mode, but
the echo mode. For the dialup applications discussed in this
book, you will be dealing exclusively with the echo mode.)
Speed
There is a lot of confusion about modem transmission speed.
I strongly suspect this is a result of the fact that it's easier
to say "baud" than "bits per second," though misinformation has a
hand in it, too.
If you've ever found yourself confused by the relationship
of bits and bauds, or if you think that a modem's baud rate is
the same as bits or characters per second, please read this
section carefully; I guarantee to clear up the confusion and
disabuse you of any false concepts ...
Bits per second (bps). Bits per second is a measure of the
number of bits transmitted each second in a communications
channel. This is sometimes referred to as "bit rate."
While a modem's bit rate is tied to its baud rate, the two
are not the same, as explained below.
Baud rate. Baud rate is a measure of the number of times
per second a signal in a communications channel varies, or makes
a transition between states (states being frequencies, voltage
levels, or phase angles). One baud is one such change; thus, a
300 baud modem's signal changes state 300 times each second.
Determining bits per second. Depending on the number of
states used in a communications system, one change of state can
transmit one bit--or more or less than one bit. The number of
bits a modem transmits per second is directly related to the
number of bauds that occur each second, but the numbers are not
necessarily the same.
To illustrate this, first consider a modem with a baud rate
of 300, using a transmission technique in which one change in
state (baud) transmits one bit. The bps rate is also 300:
300 bauds per second X 1 bit per baud = 300 bps
This describes exactly the process used by FSK--the
modulation technique used with 300 baud modems that conform to
the Bell 103 standard. Only one change in state--from ON to OFF-
-is required to send a bit when FSK is used.
Similarly, if a modem operating at 1200 baud were to use one
change in state to send each bit, that modem's bps rate would be
1200. (There are no 1200 baud modems, by the way; remember that.
This is only a demonstrative and hypothetical example.)
Now, consider a 1200 baud modem using a modulation technique
that requires two changes in state to send one bit, which can
also be viewed as 1/2 bit per baud. This modem's bps rate is
only 600:
1200 bauds per second X 1/2 baud per bit = 600 bps
To look at it another way, bits per second can also be
obtained by dividing the modem's baud rate by the number of
changes in state, or bauds, required to send one bit:
1200 baud
--------------- = 600 bps
2 bauds per bit
Lest I mislead you into thinking that "any 1200 baud modem"
should be able to operate at 2400 bps with a two-bits-per-baud
modulation technique, remember that I said there are no 1200 baud
modems. Medium- and high-speed modems use baud rates that are
lower than their bps rates. Along with this, however, they use
multiple-state modulation to send more than one bit per baud.
For example, 1200 bps modems that conform to the Bell 212A
standard (which includes most 1200 bps modems used in the U.S.)
use a phase modulation technique that transmits four bits per
baud. Such modems are capable of 1200 bps operation, but not
2400 bps because they are not 1200 baud modems; they use a baud
rate of 300. So:
300 baud X 4 bits per baud = 1200 bps
or
300 baud
------------------ = 1200 bps
1/4 baud per bit
Similarly, 2400 bps modems that conform to the CCITT V.22
recommendation (virtually all of them) actually use a baud rate
of 600 when they operate at 2400 bps. However, they also use a
modulation technique that transmits four bits per baud:
600 baud X 4 bits per baud = 2400 bps
or
600 baud
------------------ = 2400 bps
1/4 baud per bit
Thus, a 1200 bps modem is not a 1200 baud modem, nor is a
2400 bps modem a 2400 baud modem.
To continue this demonstration, 9600 bps modems operate at
2400 baud, but use a modulation technique that yields four bits
per baud:
2400 baud X 4 bits per baud = 9600 bps
or
2400 baud
------------------ = 9600 bps
1/4 baud per bit
#
If nothing else, I hope the examples here have shown you
just why baud rate is not always equivalent to bps rate. (And if
anyone who tries to sell a modem to you tells you otherwise,
you'll do well to take your business elsewhere.) When you're
considering a particular modem for purchase, look for its bps
rate, rather than its baud rate.
Characters per second (cps). Characters per second is the
number of characters (letters, numbers, spaces, and symbols)
transmitted over a communications channel in one second. Cps is
the bottom line in rating data transmission speed, and a more
convenient way of thinking about data transfer than bauds or
bits.
Determining the number of characters transmitted per second
is easy: simply divide the bps rate by the number of bits per
character. You must of course take into account the fact that
more than just the bits that make up the binary digit
representing a character are transmitted when a character is sent
from one system to another. As illustrated a few pages back in
Figure 3.16, up to 10 bits may be transmitted for each character,
whether 7 or 8 data bits are used.
In asynchronous data communication, the number of bits per
character is usually 10 (either 7 data bits, plus a parity bit,
plus a start bit and a stop bit, or 8 data bits plus a start bit
and a stop bit). Thus:
300 bps
----------------------- = 30 characters per second
10 bits per character
1200 bps
----------------------- = 120 characters per second
10 bits per character
2400 bps
----------------------- = 240 characters per second
10 bits per character
Common speeds. The most commonly-used communications rates
for dial-up systems (and hence the most commonly available modem
speeds) are 300, 1200, and 2400 bps. A few older systems--
especially Telex systems--communicate at 110 bps, but these are
gradually going the way of the dinosaur. 4800 and 9600 bps
modems are generally available, but few online services or BBSs
accommodate them. This will change in the near future, however,
with the cost of high-speed modem technology decreasing as the
demand for it increases.
Modems with even higher bps rates are manufactured (19,200
and up) but it is doubtful that you will be accessing dial-up
systems capable of more than 9600 bps (if that). The upper limit
on asynchronous data transmission via voice-grade telephone lines
appears to be 9600 bps. The use of higher transmission rates
requires special dedicated lines that are "conditioned" (i.e.,
shielded from outside interference) as well as expensive
modulation and transmission equipment.
#
Those are the basics of what goes on behind the scenes in
data communications. If you've read all of this chapter (and I
hope you have), my congratulations; you're now more knowledgeable
in the ins and outs of data communications than the average modem
user (and most computer salespeople, for that matter). The
knowledge you've gained here will enable you to make optimum use
of your modem and communications software. Combined with the
information in the next chapter, it will also help you make an
informed decision when you buy a modem.
#
If you found this excerpt useful, you may want to pick up a
copy of the book from which it was excerpted:
THE MODEM REFERENCE
by Michael A. Banks
Published by Brady Books/Simon & Schuster
ISBN # 0-13-586646-4 $19.95
In addition to explaining the technical aspects of modem
operation, communications software, data links, and other
elements of computer communications, the book provides detailed,
illustrated "tours" of major online services such as UNISON,
CompuServe, DELPHI, BIX, Dow Jones News/Retrieval, MCI Mail, and
others. It contains information on using packet switching
networks and BBSs, as well as dial-up numbers for various
networks and BBSs, and the illustrations alluded to in this
excerpt.
You'll also find hands-on guides to buying, setting up,
using, and troubleshooting computer communications hardware and
software. (And the book "supports" all major microcomputer
brands.)
For more information, contact:
Michael A. Banks
P.O. Box 312
Milford, OH 45150